An input value from a camera is nonlinearly converted into a converted value and an image based on the converted value is displayed by a monitor. It is requested to increase the speed of arithmetic processing of circuitry that performs the nonlinear conversion and reduce a memory capacity. Therefore, reference converted values of reference input values at predetermined intervals are stored in a memory as a look-up table. The converted value of an input value between reference input values is computed by linear interpolation using those reference converted values read from the look-up table.
However, when the linear interpolation is performed, the detection of the two reference input values corresponding to an input value is needed. It is likely that a longer calculation time is consumed for the detection processing.